About the connection of the Audio PC (which plays the audio) to the network :

WiFi forbidden in the Audio PC (well, we like the better SQ, right ?). This means that the Audio PC will be wired-connected to something we can call the LAN (Local Area Network) and in that LAN there's a WiFi Access Point somewhere.

Notice that this isn't about the LAN as such at all; it is only about a connection to that WiFi Access point. Most have such a WiFi access point these days and often it is built-in in the Internet Router (the box which connects to the outer world).So doing it like this, theoretically connects your Audio PC to the Internet (not really because it requires settings, but alas).

When you get yourself an explicit WiFi Access Point ($20 or so), meant to connect to a LAN to provide WiFi (communications) then you can also connect that Access Point with a direct Ethernet cable to the Audio PC. This now never allows Internet access (just don't stick any other Ethernet cables in it and also don't "bridge" the Access Point with the other WiFi you may/will have).

This is now hardware-wise ready for your Tablet etc. to communicate with the Audio PC.